1. Field of the Invention
The present invention relates to an active matrix thin film transistor (“TFT”) organic light emitting diode (“OLED”) display and a method of fabricating the same. More particularly, the present invention relates to an OLED having low leakage current, improved response time, simple structure, and high reliability, and a method of fabricating the same.
2. Description of the Related Art
In general, an active color image display device using an OLED includes a switching (sampling) transistor, a memory capacitor, and a driving transistor which controls a current supplied to the OLED according to an image signal voltage applied to the memory capacitor. A specific example of related art is a two transistors-one capacitor (“2T-1C”) structure disclosed in Japanese Patent Laid-Open Gazette No. 2002-156923.
Channels for the switching and the driving transistors are generally formed of amorphous silicon (“a-Si”) or polycrystalline silicon (“POLY-Si” or “p-Si”). The switching transistor is a switching element which supplies a data voltage to the driving transistor and thus requires a low leakage voltage and fast response. The driving transistor supplies a current to an OLED and thus requires high reliability while sustaining a high current for a long period of time.
Amorphous silicon has low electron mobility which degrades high-speed operation, making amorphous silicon unattractive for use as a material which forms the switching transistor. In addition, amorphous silicon abruptly degrades under high current operation. Thus, amorphous silicon is not suitable as a material for forming the driving transistor.
In contrast, polycrystalline silicon has high electron mobility, providing improved high speed performance. Polycrystalline silicon is also considerably less susceptible than amorphous silicon to degradation under high current. Thus, polycrystalline silicon is preferred to amorphous silicon as a material which forms the switching and driving transistors.
Unfortunately, polycrystalline silicon generates an off-current due to a leakage of a current through grain boundaries. Furthermore, polycrystalline silicon has low crystal line uniformity which makes it difficult to obtain a uniform operating characteristic for each pixel. To compensate for the low crystal line uniformity of polycrystalline silicon, a voltage program type compensation device (e.g., as made by Sarnoff, SID 98), a current program type compensation device (e.g., as made by Sony, SID 01), or other similar device is required. Such compensation devices require complicated circuits which are difficult to design and manufacture. Such compensation devices also introduce new problems.
Therefore, it is desirable to develop an organic electro-luminescent display having advantages of low leakage current, fast response time, simple structure, and high reliability without requiring the use of compensating circuits or other similar devices.